Endomixis and size variations in pure lines of Paramecium aurelia

Abstract

Jennings 1 considers the mean size of a pure line as strictly hereditary throughout the pure line; it belongs to one of the fundamental characteristics of this individual pure line. Eight years later the same author 1 reverses his opinion and based on extensive studies, claims that a single stock, i. e., a pure line derived by fission from a single progenitor gradually differentiates into such hereditarily diverse stocks; so that by selection marked results are produced. His former conception of the genotype was gained by experiments with the highly specialized infusorian species Paramecium caudalum and aurelia, his later ideas by his results with Difflugia corona, an amæbina of simpler cytological structure.

Diflugia has definite structural characters that can be counted and measured, which are unchanged by growth and environmental conditions, but still are “hereditable, yet variable.” This coincidence of favorable conditions-besides theoretical considerations-gives a priori more support to views that do not maintain the absolute constancy of the genotype, though they contradict the current conception of the “so-called” pure line. Paramecium is changed in size by its daily divisions and by the influences of the environment, thus complicating genetic studies; Jennings's results have been challenged by Walton 2 and Castle, 3 as statistically considered far from conclusive. The mean length, a constant after his opinion, varies between 114.033, 123.606, 130.120 and 144.880 microns for mass cultures, derived from the same animal. A new complication arose after the discovery of endomixis, 4 a dynamic, periodic reorganization process, involving the disintegration and absorption of the old macronuclear and micronuclear material without the introduction of foreign nuclear material. It was evident that the discrepancies in Jennings's measurements might be explained, if there are periodically appearing fluctuations during the intermictic periods.